1. Field of the Invention
The present invention relates to an image forming apparatus such as the electrophotographic copying machine and printer which use the electrophotographic engineering and, more particularly, it relates to a two-color image forming apparatus provided with a bipolar photosensitive body.
2. Prior Art and Its Problems
It was conventionally common that the two-color image forming apparatus in which the bipolar photosensitive body was incorporated used a corona discharge unit shown in FIG. 1 to achieve primary charge and secondary charge, which is reverse in polarity to the primary charge.
The corona discharge unit comprises a corona wire 1 and a conductive shield 2 and serves to apply high voltage to the corona wire 1 to ionize the circumjacent atmospher. This gas thus ionized advances to the opposite pole, that is, a grounded photosensitive body 3 according to polarity to charge the surface of the photosensitive body 3. However, corona ion flow emitted from the corona discharge unit is radial as shown in FIG. 1 and its speed is insufficient. When a secondary charge of reverse polarity is applied, using the corona discharge unit, onto the photosensitive body 3 which has a first image of a predetermined polarity, therefore, corona ions of reverse polarity concentrate onto edge and fine line portions of the first image which are large in potential contrast, and the amount of charge at these portions reaches the bias level of second development, thereby causing first and second image toners to be mixed at the time of the second development.
FIG. 2 shows a model of the process attained when the corona discharge unit shown in FIG. 1 is used as a secondary charge unit for the two-color image copying apparatus.
Step (i) represents a step by which primary charge of a predetermined polarity is applied onto a bipolar photosensitive body P. First light exposure is applied to an original OR (or image of two colors A and B on white C) through a filter, which shields the color A but allows the color B to pass through, at a step (ii) to form a first electrostatic latent image. Step (iii) represents a first developing step by which a toner (a) of the color A is stuck to those portions which have not been exposed at the time of the first light exposure, to thereby form a first image. A step (iv) represents secondary charge which is reverse in polarity to the first charge. Since corona ions at the time of the secondary charge concentrate onto the edge portion of the first image which is large in potential contrast, as described above, even potential at those portions where the color-A toner (a) has been stuck is inverted to positive polarity which corresponds to the polarity of the secondary charge, and it reaches bias level at the time of second development in which a toner (b) of the color B is used. As the result, the color-B toner (b) mounts on those portions which have been developed with the color-A toner (a) to thereby cause mixture of colors, when second development is carried out using the color-B toner (b) after second light exposure is done using the filter which shields at least the color B, as shown by a step (v). This is outstanding at the fine line and edge portions of the first image.
In order to solve this problem, it is supposed that a corona discharge unit shown in FIG. 3 is used as the secondary charge unit.
The corona discharge unit comprises a corona wire 1, conductive shield 2, and insulating slit member 4. The slit member 4 is arranged at the opening of the conductive shield 2 to allow corona ions to be emitted only through the slit thereof.
This corona discharge unit improves the straightly-advancing capacity of corona ion flow to some extent, but the angle of emitting the so-called corona ions is only made narrower. In the case where the corona discharge unit is used at the secondary charge of reverse polarity by means of the two-color image forming apparatus such as the two-color image copying machine in which the bipolar photosensitive body is incorporated, therefore, mixture of colors which is caused at the edge and fine line portions of the first image at the time of developing the second electrostatic latent image cannot be prevented completely.
In the case of the corona discharge unit shown in FIG. 3, corona ions which contribute to charging are only those which are emitted through the slit of the slit member 4, thereby leaving charging effect insufficient.